Depression is a physical illness caused by a faulty immune system that may be treated using anti-inflammatory drugs, scientists say.
Current treatments for depression focus on restoring serotonin and other mood-boosting chemicals in the brain.
However, researchers from the University of Cambridge in the UK have found that an overactive immune system triggers inflammation throughout the entire body, sparking feelings of hopelessness, unhappiness and fatigue.
It may be a symptom of the immune system failing to switch off after a trauma or illness, and is a similar to the low mood people often experienced when they are fighting a virus, like flu.
Recent studies and clinical trials have shown that treating inflammation seems to alleviate depression, researchers said.
The immune system triggers an inflammatory response when it feels it is under threat, sparking wide-ranging changes in the body such as increasing red blood cells, in anticipation that it may need to heal a wound soon.